The invention relates to a relay connection with at least two relays connected in series or in parallel to a supply voltage. In addition, the invention relates to a monitoring circuit for monitoring at least one signal input, such as an emergency stop signal, with at least two relays, which are connected in series or in parallel to a supply voltage and are controllable into an active or passive state in dependence on a signal applied at the at least one signal input.
A monitoring circuit of the abovementioned type is described in both DE-A-197 15 098 and DE-A-197 15 013.
Both cases are concerned with a safety and/or monitoring circuit for protecting a machine with several signal inputs that are monitored continuously and, if a fault occurs, the power supply to a machine is interrupted. The safety and/or monitoring circuit comprises at least two relays, which are controllable and connectable via the safety and/or monitoring circuit, as well as a start-up switch. The identical relays are series-connected and are started-up one after the other. A capacitor, the charge of which directly controls the relay, is positioned parallel to the relay to be started-up first, at least one transistor being series-connected to each of the relays, a first relay being connected to the base and a second relay connected to the collector of the transistor.
In the case of this type of circuit arrangement, identical relays, more especially with identical inductive resistance and/or identical minimum holding voltage, are used. If there is a short-term voltage dip, one of the relays may be dropped, whilst the other, however, remains operational so that the circuit arrangement has to be transferred manually into a normal state.
DE-C-43 37 665 discloses a contactor safety combination, which, through efficient utilisation of the switching members, enables a release circuit to be realised with three four-poled auxiliary contactors. In the case of the contactor safety combination, the safety or emergency stop contact is connected to the coils of the auxiliary contactors connected to the control voltage at one end. The series circuit formed by the break contacts of the second and third auxiliary contactors is connected upstream of the first coil and a series circuit formed by the make contacts of the second and third auxiliary contactors and a break contact of the first auxiliary contactor are connected upstream of the second and third parallel-connected coils. Parallel to the said series circuit, a make contact of the first auxiliary contactor is series-connected to a break contact of an ON switch.
DE-C-197 22 927 discloses a circuit arrangement with a safety function with at least two safety relays, which are connectable to a supply voltage via at least one emergency switch and the operating contacts of which are located in at least one release current circuit. In the normal operating state, the safety relays are automatically connected after excitation via a start-up switch. A capacitor is associated with each safety relay. The contacts of the safety relays are realised and positioned in such a manner that when the start-up switch is closed, one of the safety relays is excited and the capacitors are put to supply voltage and are charged, when the start-up switch is subsequently opened, the one safety relay is de-energised and the other safety relay is excited by the charging capacitor associated therewith, whereupon the one safety relay is excited by the charging capacitor associated therewith and both safety circuits move into automatic mode.
DE-A-102 16 226 discloses a device for the fail-safe disconnection of an electric consumer, more especially in industrial production plants. In a safety circuit, two contactors are connected in parallel to a single-channel connecting lead. The second connection of the contactors is connected to reference potential separately from the first safety switching device.
U.S. Pat. No. 6,236,553 describes a safety relay, which has at least one input, which is monitored for closed and open state and is connectable to the safety device and actuator, and at least one output for connection to the machine or the process, one or several stop relays for generating an interrupt and a reset circuit with at least one capacitor and one reset input. The capacitors receive and store electric power in a first state, whilst in a second state they output the power in order to reset the safety relay into a normal operating state. The safety relay also has first means, which are connected to the capacitor, which, in a first state, establish an enclosed current path between a first and a second supply voltage for charging the capacitor and, in a second state, isolate the first means from the second supply connection. Second means are positioned parallel to the capacitor for conducting current, in a second state, from the capacitor via a second supply connection to the relay coil in the stop relay in order to reset the safety relay.
DE-A-199 13 933 relates to a circuit arrangement and a method for connecting a consumer, the circuit arrangement having the following features:—a start-up circuit having a switching means for preparing at least one first control signal that is dependent on a change in a switching position of the switching means and a second control signal that is dependent on an actuating duration of the switching means;—at least one release circuit with a first control element of a first switching device having contacts and with a first switching means series-connected to the first control element for connecting the control element to a supply voltage according to the first and second control signals.
A switching arrangement with safety function is disclosed in DE-A-197 51 674, for a safety circuit with at least two relays, which are connected to a supply voltage and the operating contacts of which are inserted in at least one release current circuit.
Exclusively identical relays or respectively relay coils are not used in any of the above-mentioned citations.